Since the table is network-wide, this makes sense to avoid duplicating
options and calling dbDelta() continuously.

This works even if the image is not a 'post thumbnail' image.

Fixes issue #1.

And replace get_post_permalink() with the more generic
get_permalink() function.

The template had problems with string quoting (PHP
only allows interpolation in double-quoted strings!).

The SQL schema changes the primary key,
so we're switching to a different table to avoid
having to dedupe existing data.